In my prior copending application Ser. No. 37,537, filed Apr. 13, 1987, polyimide precursor compositions are described which are eminently useful in psuedo hot-melt prepregging processes. These compositions are composed of a mixture of (a) about 40 to about 70 weight percent of a substantially equimolar mixture of (i) 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane or 2,2-bis[4-(3-aminophenoxy)phenyl]hexafluoropropane, or both, and (ii) lower alkyl diester of at least one aromatic tetracarboxylic acid; (b) about 2 to about 50 weight percent of polyamide producible from components (i) and (ii); and (c) about 10 to about 50 weight percent of solvent. Such mixtures are clear resinous solids or semi-solids at 25.degree. C. They have softening temperatures in the range of 30.degree. to 40.degree. C. At 60.degree. C. they exist as viscous liquids having a viscosity in the range of 2,000 to 12,000 centistokes.
There are instances where it is desired to transform these or other non-flowable or essentially non-flowable polyimide precursor compositions into flowable liquids that can be readily poured from a container in which they are shipped or stored. While it is possible to add solvent to the compositions to render them free-flowing, this is undesirable as the compositions are usually employed for their intended purposes in the form of systems of high solids content. Another way of transforming the compositions into flowable liquids is to heat them to a suitable temperature at which their viscosity is decreased. However because of the reactivity of the mixture, it is necessary to keep the temperature relatively low, otherwise the mixture will undergo chemical transformations--viz., amidization and/or imidization.
Complicating the problem is the fact that heat transfer through these viscous compositions is poor. Thus, when dealing with, say, pound quantities or more, attempts to fluidize the material by heating in an oven even to relatively low temperatures (e.g., 60.degree. C.) results in local overheating and consequent amidization and imidization. In the case of precursors for prepregging such chemical transformations can render the precursor unsuitable for use, as by forming prepregs with too little tack and drape.
The need for close control of the composition of prepreg precursors and the avoidance of undesired chemical transformations therein prior to prepreg formation is illustrated by the fact that such precursor compositions are normally kept in a frozen state to prevent the premature onset of amidization and worse yet, imidization. Moreover, the product and performance specifications imposed on such materials tolerate little premature change in composition.